An Introduction to Statistical Learning: With Applications in R, Gareth James, Daniela Witten, Trevor Hastie, Rob Tibshirani, 2021 (Springer) - This book is a widely used resource for statistical learning, explaining regression metrics like RMSE and their role in model assessment. (2nd edition)
Machine Learning (CS229) Lecture Notes, Andrew Ng, 2023 (Stanford University) - These widely recognized lecture notes offer a basic treatment of machine learning, including discussions of regression problems and associated error metrics.